2. TERMINOLOGY
NOMINAL SIZE: It is the size of a
part specified in the drawing.
BASIC SIZE: It is the size of a
part to which all limits of
variation are determined.
ACTUAL SIZE: It is the actual
measured dimension of a part.
Nominal and basic size are often
the same.
3. LIMIT OF SIZES
There are two
extreme possible
sizes of a
component.
The largest
permissible size
for a component is
called upper limit
and smallest size is
called lower limit.
4. DEVIATION
It is the algebraic
difference between
any given size and
actual size.
ACTUAL DEVIATION: It
is the algebraic
difference between
the actual size and
the basic size.
5. DEVIATION
LOWER DEVIATION:
It is the algebraic
difference
between the
minimum limit of
size and the basic
size.
UPPER DEVIATION:
It is the algebraic
difference
between the
maximum limit
and the basic size.
6. ZERO LINE
It is the
straight line
corresponding
to the basic
size. The
deviations are
measured from
this line.
7.
8. NATURAL VARIABILITY OF
PROCESS
It is the variation occurred in the size due to
natural conditions like variations in material,
environmental fluctuations, vibrations,
human variability etc. It is an unavoidable
process.
9. POSITIONAL TOLERANCES
Two types of positional tolerances are used:
1. Unilateral tolerances
2. Bilateral tolerances
When tolerance is on one side of basic size,
it is called unilateral and if it is both in plus
and minus then it is known as bilateral
tolerance.
10. FITS
The degree of
tightness or
looseness between
two mating parts is
called a fit.
11. TYPES OF FITS
CLEARANCE FIT:
There is a
clearance or
looseness in this
type of fits. These
fits maybe slide
fit, easy sliding fit,
running fit etc.
12. TYPES OF FITS
INTERFERENCE FIT: There is an interference
or tightness in these type of fits. E.g. shrink
fit, heavy drive fit etc.
13. TYPES OF FITS
TRANSITION FIT: In
this type of fit, the
limits for the
mating parts are so
selected that either
a clearance or
interference may
occur depending
upon the actual size
of the mating parts.
14. BASIS OF LIMIT SYSTEM
1. HOLE BASIS
SYSTEM: In this
system, the hole
is kept as a
constant member
and different fits
are obtained by
varying the shaft
size.
15. BASIS OF LIMIT SYSTEM
SHAFT BASIS
SYSTEM: In this
system, the shaft
is kept as constant
member and
different fits are
obtained by
varying the hole
size.
16. DIFFERENT LIMIT SYSTEMS
1. The Newall system
2. British Standard system
3. International Federation of National
Standardization Association (ISA) system
4. ISO system
5. ISI system
17. THE TAYLOR PRINCIPLE
Taylor’s Principle lays it down:
1. A GO Gauge will check all the dimensions of the work
piece in what is called the maximum metal condition
(indicating the presence of the greatest amount of material
permitted at a prescribed surface)
2. That NOT GO Gauges shall check only one dimension of the
work piece at a time, for the minimum metal conditions
(indicating the presence of the least amount of material
permitted at a prescribed surface) size
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18. THE TAYLOR PRINCIPLE (Cont..)
In case of hole, the maximum metal
condition obtains when the hole is machined
to the low limit of size, & minimum metal
condition results when the hole is made to
the high limit of size.
in case of shaft the limits taken would be
inverse of hole
